Influence of load ratio on near-threshold fatigue crack propagation behavior

In 1970, Elber found crack closure in Stage II fatigue crack growth. Since then, a considerable amount of work has been conducted regarding the influence of crack closure on Stage II fatigue crack propagation behavior. It was reported that the concept of crack closure could be used to explain the effects of load ratio (R=sigma /SUB min/ /sigma /SUB max/ ) and overload on fatigue crack growth rates. Recently, Ritchie, Stewart, Minakawa and McEvily have demonstrated the importance of crack closure during Stage I (near-threshold) crack growth. It was found that plasticity, oxide and roughness-induced crack closure played a crucial role in understanding near-threshold crack propagation behavior. In this paper, the influence of R-ratio on threshold crack growth rates is discussed in light of crack closure